A spark plug for an internal combustion engine includes a cylindrical housing, a cylindrical insulator that is supported inside the housing, a center electrode supported inside of the insulator so as a distal end portion thereof protrudes, and a ground electrode that forms a spark discharge gap between the ground electrode and the center electrode.
In such a spark plug, with a spark discharge that occurs in the above-mentioned spark discharge gap, radio noise is generated from the center electrode, and this may affect peripheral equipment.
In order to improve a capability of preventing this radio noise (noise suppression performance), there is known a device to which a resistor is disposed on a proximal side of the center electrode (refer to Japanese Patent Publication No. 4901990, for example).
However, there are following problems to the spark plug for the internal combustion engine mentioned above.
In recent years, for the purpose of improving fuel consumption of the internal combustion engine, the adoption of supercharging or the increasing of the compression ratio has been studied.
Accordingly, there is a tendency that the temperature in a combustion chamber increases.
In this case, the temperature of a distal end portion of the spark plug exposed to the combustion chamber is likely to be high, and the heat of the distal end portion is easily transmitted from the center electrode to the resistor disposed in the proximal side.
Therefore, the temperature of the resistor is also likely to be high.
Accordingly, materials constituting the resistor are easily oxidized, thus there is a risk that a resistance value of the resistor may increase.
As a result, electric discharge sparks are not easily generated, and this can lead to a misfire in the internal combustion engine.
Here, in order to prevent the resistors from easily becoming hot, it is considered to keep the resistor away from the distal end of the center electrode to the proximal side.
However, from a viewpoint of noise suppression performance, it is not preferable to keep the resistor away from the distal end of the center electrode.